This invention relates to systems for resisting thermal extremes and flame. It has particular application to systems which protect underlying substrates. It also relates to methods of providing such protection.
The situations in which it is desirable to protect substrates from heat and flame include, for example, protecting static structures such as petroleum storage tanks, chemical production equipment, electrical cable trays, and structural steel from the spread of fire and protecting transportation equipment such as tank cars and aircraft cabins from the same risks.
Numerous thermal protective coating compositions and systems for applying them are known. Some of the compositions are foamed passive insulative compositions which protect merely by their low thermal conductivity and their thickness as applied. These include foamed cement or intumesced silicates. The present invention is not concerned with such systems.
Other compositions provide active thermal protection. Some intumesce when heated to form a thick closed cell protective layer over the substrate. These include silicate solutions or ammonium phosphate paints or compositions such as those disclosed in Nielsen et al., U.S. Pat. No. 2,680,077, or Kaplan, U.S. Pat. No. 3,284,216. Other active thermal protective compositions include constituents which sublime at a predetermined temperature, such as those disclosed in Feldman, U.S. Pat. No. 3,022,190. The active thermal protective compositions disclosed in Feldman, U.S. Pat. No. 3,849,178 are particularly effective; when subjected to thermal extremes, these compositions both undergo an endothermic phase change and expand to form a continuous porosity matrix. The term "active" is applied herein to indicate thermal protective compositions which respond to thermal extremes either by expanding or by undergoing a highly endothermic change, or both.
Various methods and structures have also been used or proposed for applying these thermal protective coating compositions. The most frequent approach is to apply the compositions directly to the substrate, without additional structure. For many applications, however, a reinforcing material, such as fiberglass sheet or a wire mesh, has been embedded in the coating composition to strengthen the composition and prevent it from cracking or falling off the substrate under conditions of extreme vibration, such as explosion or earthquake, or under conditions of flame or thermal extreme. Reinforcement has been found to be particularly important when the thickness of the composition exceeds about one-quarter inch (4 mm) or when the composition is rated at more than about one-half hour in a standard fire test. Examples of this approach are found in Feldman, U.S. Pat. No. 3,022,190, Billing et al, U.S. Pat. No. 3,913,290, Kaplan. U.S. Pat. No. 3,915,777, and Billing et al, U.S. Pat. No. 4,069,075. Sometimes the compositions are first applied to a reinforcing structure such as a flexible tape or flexible wire mesh, and the combined structure is applied to the substrate. Examples of this approach are found in Feldman, U.S. Pat. No. 3,022,190, Pedlow, U.S. Pat. No. 4,018,962, Peterson et al. U.S. Pat. 4,064,359, Castle, U.S. Pat. No. 4,276,332, Fryer et al, U.S. Pat. No. 4,292,358, and Feldman, U.S. Pat. No. 4,493,945. In these last-mentioned systems, the purpose of the reinforcing structure may be both to strengthen the resulting composite and to permit its application to a substrate without directly spraying, troweling or painting the uncured coating compositions onto the substrate.
In any of the foregoing methods and structures, multiple layers are frequently applied to the substrate to provide additional protection.
The use of reinforcing structures in active thermal protective compositions, however, has not been completely successful. Fiberglass sheet embedded in active thermal protective compositions has been found to embrittle with heat, and the composition in which it is embedded may crack and fail under fire conditions. Silicone fabrics suffer similar problems. Metal mesh is difficult to apply; it generally requires studs to mount it on the underlying substrate, and the stiffness of the mesh makes it difficult to form and to work with. Presently known systems and methods, moreover, are not as efficient, in terms of length of protection for a given weight of protective composition, as desirable. Efficiency is particularly important because in many applications weight or volume is critically limited.